Human Physiology, 14th edition (2016)

The Immune System 507

15.3 FUNCTIONS OF

T LYMPHOCYTES

Killer T cells effect cell-mediated destruction of spe-

cific victim cells, and helper and suppressor T cells play

supporting roles. T cells are activated only by antigens

presented to them on the surface of particular antigen-

presenting cells. Activated helper T cells produce lympho-

kines that stimulate other cells of the immune system.

complement proteins, not the antibodies directly, kill the cell;
antibodies serve only as activators of this process in the classic
pathway.
Complement fragments that are liberated into the sur-
rounding fluid rather than becoming fixed have a number of
effects. These effects include (1) chemotaxis —the liberated
complement fragments attract phagocytic cells to the site
of complement activation; (2) opsonization —phagocytic
cells have receptors for C3 b , so that this fragment may form
bridges between the phagocyte and the victim cell, thus
facilitating phagocytosis; and (3) stimulation of the release
of histamine from mast cells and basophils by fragments
C3 a and C5 a. As a result of histamine release, blood flow
to the infected area is increased because of vasodilation
and increased capillary permeability. This helps to bring
in more phagocytic cells to combat the infection, but the
increased capillary permeability can also result in edema
through leakage of plasma proteins into the surrounding tis-
sue fluid.

Figure 15.11 The membrane attack complex. Fixed

complement proteins C5 through C9 assemble in the plasma

membrane of the victim cell as a membrane attack complex. This

complex forms a large pore that punctures the membrane and

thereby promotes the destruction of the cell.

C5b

C6

C7

C8

C9

| CHECKPOINT

4a. Illustrate the structure of an antibody molecule. Label

the constant and variable regions, the F (^) c and F (^) ab
parts, and the heavy and light chains.
4b. Define opsonization, and identify two types of
molecules that promote this process.

5. Describe complement fixation, and explain the
   roles of complement fragments that do not become
   fixed.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Identify the different T lymphocytes and their
   functions.


7. Explain how T cells become activated and how they
   function in immunity.

The thymus processes lymphocytes in such a way that their

functions become quite distinct from those of B cells. Lympho-

cytes residing in the thymus or originating from the thymus, or

those derived from cells that came from the thymus, are all T

lymphocytes. These cells can be distinguished from B cells by

specialized techniques. Unlike B cells, the T lymphocytes pro-

vide specific immune protection without secreting antibodies.

This is accomplished in different ways by the three subpopula-

tions of T lymphocytes.

Killer, Helper, and Regulatory

T Lymphocytes

The killer, or cytotoxic, T lymphocytes can be identified in

the laboratory by a surface molecule called CD8. Their func-

tion is to destroy body cells that harbor foreign molecules.

These are usually molecules from an invading microorganism,

but they can also be molecules produced by the cell’s genome

because of a malignant transformation, or they may simply be

body molecules that had never before been presented to the

immune system.

In contrast to the action of B lymphocytes, which kill at a

distance through humoral immunity (the secretion of antibod-

ies), killer, or cytotoxic, T lymphocytes kill their victim cells

by cell-mediated destruction. This means that they must be in

actual physical contact with the victim cells. When this occurs,

the killer cells secrete molecules called perforins and enzymes

called granzymes. The perforins enter the plasma membrane

of the victim cell and polymerize to form a very large pore.

This is similar to the pore formed by the membrane attack

complex of complement proteins, and results in the osmotic

destruction of the victim cell. The granzymes enter the victim

cell and, through the activation of caspases (enzymes involved